1. Field of the Invention
This invention relates to a lubricating oil composition for a refrigerator in which 1,1,1,2-tetrafluoroethane (HFC-134a) is used as the refrigerant.
2. Prior Art
There have heretofore been used many kinds of refrigerators, among which so-called vapor compression type refrigerators using a refrigerant which are classified into reciprocating type, centrifugal type and rotary type refrigerators depending on the structure of compressor thereof. Of these various refrigerators, those having a rotary compressor are characterized in that they have high refrigeration performances for their small size, exhibit excellent performances even in the region of high-speed rotation and are run smoothly and quietly. Thus, the refrigerators having a rotary compressor are widely used in home refrigerators and air-conditioners as well as in car air-conditioners.
The lubricating oil compositions for refrigerators (hereinafter sometimes referred to as "refrigerator oil composition(s)") used in said refrigerators and conditioners, generally include naphthenic mineral oils, paraffinic mineral oils, alkylbenzenes, polyglycolic oils and mixtures thereof, which have each a kinematic viscosity of 5-200 cSt at 40.degree. C. as well as these oils incorporated with suitable additives.
The refrigerants which are widely used for compression type refrigerators include halogen-containing refrigerants such as trichloromonofluoromethane (CFC-11), dichlorodifluoromethane (CFC-12), monochlorodifluoromethane (HCFC-22) and trichlorotrifluoroethane (CFC-113); hydrocarbon refrigerants typified by propane; and inorganic gas refrigerants typified by ammonia, carbonic acid gas and the like.
Of these refrigerants, the halogen-containing refrigerants are chemically stable, low poisonous and incombustible and have thermodynamically ideal characteristics; therefore, they are widely used in home refrigerators and air-conditioners as well as in car air-conditioners.
Of these halogen-containing refrigerants, however, perhalogenocarbon which is in the form of chlorofluorocarbon obtained by substituting all the hydrogen atoms of a hydrocarbon by halogen atoms, is difficultly decomposable and highly stable and will not decompose for a long period of time after being discharged to the atmosphere, after which the chloro-fluorocarbon will reach an ozone layer located 25-30 Km above the earth and meet with the intense ultraviolet rays emitted from the sun so as to liberate the chlorine atoms which then react with the ozone to reduce the ozone concentration in the ozone layer. The ozone layer absorbs the harmful ultraviolet rays emitted from the sun to protect the life on the surface of the earth. Thus, an international treaty which controls the production and consumption of CFCs and HCFC-22, has been proposed to be concluded in order to prevent the destruction of the ozone layer by CFCs and HCFC-22.
Under such circumstances, the U.S. Environmental Protection Agency (EPA) has proposed six kinds of substitute refrigerants which are 1,1-dichloro-2,2,2- trifluoroethane (HCFC-123), 1-chloro-1,2,2,2-tetrafluoroethane (HCFC-124), 1,2-dichloro-2,2-difluoroethane (HCFC-132b), 1-chloro-2,2,2-trifluoroethane (HFC-133a), 1,1,1,2-tetrafluoroethane (HFC-134a) and 1,1-dichloro-1-fluoroethane (HFC-141b).
Among these substitute refrigerants, 1,1,1,2-tetrafluoroethane is similar in thermodynamic properties to dichlorodifluoromethane (CFC-12) which has heretofore been widely used in home refrigerators and air-conditioners as well as in car air-conditioners. If, therefore, 1,1,1,2tetrafluoroethane (HFC-134a) is to be used as the refrigerant, then refrigerator systems using this refrigerant therein may be established without a great change in design to obtain the same refrigeration performances as conventional systems.
It is to be noted, however, that a refrigerator oil composition filled in the compressor of a refrigerator is entrained into the condenser and evaporator thereof and, therefore, it is indispensable that the oil composition be a so-called "well-returnable" oil which is fully soluble in the reflux refrigerant, circulated through the cycle and returned to the compressor.
More specifically, it is not unusual that the temperature reaches -20.degree. to -30.degree. C. at the coolest portion extending from the expansion valve or capillary tube to the evaporator within the refrigeration cycle, and, therefore, a preferable refrigerator oil composition is such that when mixed with the refrigerant, it will not separate from the refrigerant and will not form two separate layers at such a low temperature.
Further, it is not unusual that the temperature reaches a high temperature at the portion extending from the compressor to the receiver or condenser, and, therefore, a more preferable refrigerator oil composition is such that when mixed with the refrigerant, it will not separate from the refrigerant not to form two separate layers not only at the low temperature but also at a high temperature.
The refrigerator oil compositions heretofore generally used include naphthenic mineral oils, paraffinic mineral oils, alkylbenzenes and mixtures thereof as well as these base oils incorporated with suitable additives. These conventional refrigerator oil compositions are described in, for example, Japanese Pat. Gazettes Nos. Sho. 40-11940, Sho. 49-4107, Sho. 49-13483, Sho. 49-13829, Sho. 49-19084, Sho. 52-39509, Sho. 52-43722 and Sho. 53-17602, as well as Japanese Pat. Appln. Laid-Open Gazettes Nos. sho. 46-4532, sho. 48-606, sho. 49-47498, sho. 51-22971, Sho. 52-28503, sho. 52-54707 and sho. 53-88007. These conventional refrigerator oil compositions are hardly compatible with 1,1,1,2-tetrafluoroethane (HFC-134a) which is a substitute refrigerant for dichlorodifluoromethane (CFC-12). For this reason, if the conventional oil compositions are used in combination with 1,1,1,2-tetrafluoroethane then the resulting mixture will separate into two layers at normal temperature thus degrading the oil-returnability which is most important within the refrigeration system, with the result that there are caused various troubles such as a decrease in refrigeration efficiency, the deterioration of lubricity and the consequent seizure of compressor whereby the oil compositions are made unsuitable for use as such. In addition, refrigerator oil compositions comprising polyglycolic oils are also known for their high viscosity index and are disclosed in, for example, Japanese Pat. Gazettes Nos. Sho. 57-42119 and Sho. 61-52880 and Japanese Pat. Appln. Laid-Open Gazette No. Sho. 57-51795. However, the polyglycolic oils concretely disclosed in these prior arts are not fully compatible with 1,1,1,2-tetrafluoroethane thereby raising the same problems as above and rendering them unusable.